There is an increasing demand for biofuels alternatives to petroleum-based fuel due to the health and environmental problems of the latter. Moreover, fossil fuel is not renewable; Campbell and Laherrere (1998) predict that petroleum reserves will be completely depleted by 2050. Recently, there has been a significant increase in the production and use of bioethanol and biodiesel. These biofuels - apart from being alternatives to fossil-derived fuels - are secure, renewable, non toxic, have a favorable energy balance and lower harmful emissions and are, therefore, environmentally friendly. Biodiesel is produced from the transesterification of vegetable oils or animal fats using simple alcohols (methanol or ethanol) and alkali catalysts. The process generates a lot of glycerol as a by-product. Specifically, the amount of glycerol generated is about 10% of the biodiesel produced (Yazdani and Gonzalez, 2008). Thus, for every 100 lb of biodiesel produced, 10 lb of glycerol is generated as waste. It is rightly predicted that crude glycerol availability will increase in the near future due to this global growth in biodiesel production (Dharmadi et al., 2006).
Crude glycerol from biodiesel production presents great economic and environmental challenges. It is expensive to purify, and its improper disposal can contaminate the lithospheric environment. Yet, its surplus collapses the price of glycerol, which affects the glycerol-producing and -refining industries. Consequently, the economic viability of biodiesel industry hangs on the balance, unless the market value of the glycerol by-product is improved. In fact, some of these industries are threatened with bankruptcy (Wilke and Vorlop 2004). Currently, glycerol-producing plants owned by some chemical companies, such as Dow chemical, and Procter and Gamble Chemicals, have been shut down (McCoy, 2006). Therefore, development of processes to convert this low-value glycerol to higher value products is an excellent opportunity to improve the economic viability of biodiesel production, and also make it environmentally safer.
Chemical and biological approaches to the conversion of biodiesel waste into high value products are currently being explored. Chemical catalysis has many disadvantages. They include: low product specificity, need for high pressure and/or temperature, and inability to use crude glycerol with high levels of contaminants (Yazdani and Gonzalez, 2007). But biological approaches through either aerobic or anaerobic fermentation hold better promise. Anaerobic fermentation is preferred over aerobic because the capital and operational costs involved in the former are less than in the later (Yazdani and Gonzalez, 2007):
Anaerobic fermenters are less expensive to build and operate than aerobic ones;
ii) Anaerobic fermenters use less energy than aerobic counterparts.
It is proposed that biofuels industries should also establish biorefinaries which convert co-products to higher value products to achieve increased economic viability (Kamm and Kamm, 2007; Yazdani and Gonzalez, 2007). A biofuel industry which has biorefinaries that convert waste into higher value products could achieve this purpose. The glycerol-rich streams of waste generated during biodiesel production have the potential to be used in the proposed technology.
Glycerol is a good carbon and energy source for many microorganisms, and, therefore, can be an invaluable feedstock for industrial fermentations (Da Silva et al., 2009). The ability to ferment glycerol in the absence of air is found in a few members of Enterobacteriaceae (Bouvet et al., 1995). Fewer members of this family of bacteria namely Citrobacter, Klebsiella, Enterobacter, and Escherichia have been reported to produce ethanol as a major product of anaerobic fermentation of glycerol (Gonzalez et al., 2008; Homann et al., 1990; Ito et al., 2005; Jarvis et al., 1997; Lin, 1976; Streekstra et al. 1987). Other co-products include hydrogen, 1, 3-propanediol, succinate, lactate, acetate, propionate, formate, and 2, 3-butanediol. However, the ethanol production was very slow and the quantity too low in all but one of the reported cases. Enterobacter aerogenes is the only species reported to produce ethanol, hydrogen and carbon dioxide as the main products (Ito et al., 2005). It is a facultative anaerobe and can be utilized for high-yield production of ethanol from crude glycerol. The biological fermentation of glycerol into ethanol and H2 is attractive because H2 is expected to be a future clean energy source while ethanol can be used as raw material, a supplement to gasoline, and a feedstock for biodiesel production in place of methanol (Sakai and Yagishita, 2007).
The maximum theoretical yield of ethanol and hydrogen (or formate) from glycerol dissimilation is 1 mol each of ethanol and hydrogen (or formic acid) per mol of glycerol utilized.
C3H8O3 -----------> C2H5OH + H2 + CO2
C3H8O3 -----------> C2H5OH + HCOOH
This means that 50% glycerol is theoretically converted to ethanol, 2.2% is converted to H2, and 47.8% is converted to CO2; or 50% glycerol is converted to ethanol, and 50% is converted to formic acid.
This paper presents the result of research using E. aerogenes ATCC 13048 to ferment pure (P-) and recovered (R-) glycerol into ethanol. It also discusses the development of a mutant strain of the named bacterium, which is capable of growing in a high glycerol concentration and of resisting product (ethanol) inhibition. Finally, this paper reports the result of using the new strain to convert P-glycerol to ethanol vis-à-vis the original strain.